Most infrared detectors, such as thermistors, thermocouples, thermopiles, pyroelectric detectors, etc., are normally blackened to enhance infrared absorption. For blackening, lamp black has been used, but more modern, very flat black paints have replaced it.
It has been proposed to make selective infrared detectors which reject short wave radiation, such as, for example, visible light or near infrared. This has been done with multilayer interference filters using germanium, and while such composite detectors are operative, their cost is extremely high. For example, in instruments such as intrusion detectors, commonly called burglar alarms, the interference filter may be the most expensive part of the whole instrument; and for many uses, such as, for example, detectors for home use, the cost may become substantially prohibitive.
Intrusion detectors are well known, a typical example being shown in my earlier U.S. Pat. No. 3,760,399, Sept. 18, 1973. The detector is an all active thermocouple with strips of alternating polarity. In other words, it modulates any infrared signals received from a moving intruder. This in a rough way produces differential sensitivity because steady signals do not cause the logic circuits to respond. However, in a home, headlights from passing cars can give a false alarm.
The broad idea of reflecting short wave radiation while permitting infrared radiation to pass has been proposed for an entirely different purpose and instrument, namely a mirror for concentrating the light of a projection lamp for motion picture projection or other film projection. One of the problems is that the powerful light source also transmits large amounts of infrared radiation, which heats up the gate of the motion picture projector; and if it is attempted to increase brightness by greatly increasing light output, film can be damaged. Similarly, in the case of slide projectors the same problem arises and it is necessary to cool the film. An ingenious solution for the light concentrating mirror is shown in the U.S. Pat. No. to Turner 2,660,925, Dec. 1, 1953, in which the mirror has multiple layers, including a layer which strongly reflects visible light but permits infrared to pass through. Thus the emerging light which encounters the film gate has a large portion of the heating rays eliminated. Another general use of selective radiation is shown in the Pearson et al U.S. Pat. No. 3,445,444. A more remote application of the general principles is shown in the Jackson U.S. Pat. No. 2,721,275, Oct. 18, 1955, for detecting aircraft. in this case, instead of preventing short wave radiation from striking the detector, a separate detector receives visible light from the sky or other sources and in the electronic amplifiers this signal opposes that of the longer wavelength signal from the aircraft itself, usually the exhaust of the aircraft motor. In other words, the effect of short wave radiation is balanced out electrically. Such a system, of course, is not very practical for an intrusion detector and is also quite expensive. It is with a low cost, simple instrument and detector, especially for intrusion detectors, that the present invention deals.